Lead-acid batteries serve as back-up power supplies for electrical equipment in industrial and commercial systems requiring uninterrupted operation. Telephone switching systems and computer back up systems are examples of systems that use lead acid batteries to provide stand-by power in case of interruption of public utility service.
As used herein, the following terms to have the following meanings:
"Cell": An assembly of a positive plate and a negative plate, spaced apart from each other, which produces an electrochemical reaction, manifesting itself as a voltage potential between the two plates, when an electrolyte containing sulfuric acid is introduced into the cell. The cell may contain a separator located between the positive and negative plates. PA0 "Positive plate": A metal plate, within a cell, containing an active material that is electrochemically reduced during cell discharge. PA0 "Negative plate": A metal plate, within a cell, containing an active material that is electrochemically oxidized during cell discharge. PA0 "Electrolyte": A solution which transports electrons and which, in some cases, participates in reactions at the positive and negative plates. PA0 "Battery": A device containing a cell, or a group of cells connected in series, and having positive and negative terminals. PA0 "Half-cell reaction": An electrochemical reaction occurring at either the positive or negative plate in the presence of electrolyte. PA0 "Thermodynamic equilibrium": The state wherein flow of current through a cell or half-cell is infinitesimally small and the electrochemically active species are at unit activity. PA0 "Potential": The electromotive force, measured as volts associated with an electrochemical reaction or combination of reactions. PA0 "Standard potential": The thermodynamic equilibrium potential of a half-cell reduction reaction as measured by the standard hydrogen electrode. PA0 "Polarizations": The deviation in potential, from the standard potential, of a half-cell reaction caused by flow of current through the half-cell. Polarization typically changes as current through the half cell changes. PA0 "Recombinant cell": A cell in which, during charging or recharging, oxygen produced at the positive plate by hydrolysis of water reacts with lead at the negative plate to produce lead oxide which in turn is reduced to metallic lead, liberating oxygen which in turn forms water.
In lead-acid batteries, the positive plate is made by filling the spaces of metallic lead or lead alloy grid with lead sulfate paste. The lead sulfate paste converts electrochemically to PbO.sub.2 during battery "forming" or initial charging. During this process, the surface of the lead or lead alloy grid reacts with electrolyte, creating a film of PbO.sub.2 on the grid. Thus the positive plate has an interior grid of metallic lead or lead alloy, with PbO.sub.2 in the spaces of the grid.
The negative plate is made by filling the spaces of a lead or lead alloy grid with lead sulfate paste. During battery forming or initial charging, the lead sulfate paste converts electrochemically to metallic lead. Thus the negative plate contains a grid of lead or lead alloy with spongy lead in the grid spaces.
The lead sulfate paste for both the positive and negative plates may also contain various organic additives, to improve the properties of the paste or the resultant lead or PbO.sub.2.
The electrolyte for a lead acid battery contains sulfuric acid.
A typical battery cell contains an alternatives series of positive and negative plates with separators between facing positive and negative plates. Separator materials include rubber, silica-filled polyethylene, matsw made of glass, polyester and the like, and a variety of other materials.
Lead-acid batteries always have at least one cell and typically have a group of cells connected in series.
The cells are either wet cells or recombinant cells. Wet cells have excess electrolyte over and above the amount of electrolyte which is absorbed and adsorbed by the plates and separators in the cell. Recombinant cells typically have less electrolyte with the electrolyte typically being absorbed into the plates or the separator material or being gelled such as by addtion of formed silica to the electrolyte.
Potential across a cell is the sum of (i) the standard potential for the half-cell reactions at the positive and the negative plates, (ii) the positive plate polarization, (iii) the negative plate polarization and (iv) the voltage drop corresponding to the electrical resistance of the electrolyte. In many cells, voltage drop caused by resistance of the electrolyte is negligible.
Positive plate polarization is the polarization occurring at the positive plate, which is the PbO.sub.2 plate in a lead-acid battery cell. Negative plate polarization is the polarization at the negative plate, which is the lead plate in a lead-acid battery cell. Presence or absence of various elements at or in plate influences the degree of polarization for a given electrical current. Antimony or platinum in the negative plate depolarize the negative plate (i.e. presence of antimony or platinum causes potential of the negative plate to remain at the standard potential up to a higher current level than would be achieved if the antimony or platinum were not present. In recombinant cells, O.sub.2 depolarizes the negative plate.
In general, degree of polarization is proportional to the logarithm of the current flowing through the half-cell; this is known as the Tafel equation.
The degree of positive plate polarization depends on the overall potential imposed across the cell and on the amount of negative plate polarization present at the current flowing out of the battery, which resulting from the overall cell potential or voltage. It is desirable to accurately control positive plate polarization, for battery cells because positive plate polarization during float or stand-by condition, i.e. where the batter is not preparing its intended function of supply energy, in order to increase battery life.
For batteries in stand-by service, it is desirable to maintain the battery in a maximum state of readiness. Cells in batteries in an open-circuit condition i.e. having nothing connected to them, gradually self-discharge and lose their ability to perform when needed. To counteract this charging voltage (a voltage greater than the standard potential of the cell) is maintained across the cell; this counteracts the cell's tendency to self-discharge. Maintaining a cell with a charging voltage being applied in an amount just above the cell standard potential is called "floating" the cell.
At potentials above the standard potential for the reversible PbO.sub.2 /PbSO.sub.4 reaction by which lead-acid batteries operate, lead in the positive plate grid corrodes to PbO.sub.2. Since PbO.sub.2 takes up more volume than metallic lead, the positive plate grid expands with such conversion of lead to PbO.sub.2 occurring during float. If the grid expands too much, the grid loses electrical contact with the PbO.sub.2 in the grid spaces. Hence, while "floating" the cell loses its ability to perform if the positive plate grids grow excessively. Since PbO.sub.2 is the electrochemically active part of the positive plate during battery discharge, loss of electrical contact between the grid and the PbO.sub.2 results in loss of capacity for the cell or the battery containing the cell.
Lander, Ruetschi and others [J. J. Lander, "Anodic Corrosion of Lead in H.sub.2 SO.sub.4 Solution," J. Electrochem. Soc., 98 (1951) pp. 213-219; "Further Studies on the Anodic Corrosion of Lead in H.sub.2 SO.sub.4 Solutions," J. Electrochem. Soc., 103 (1956) pp. 1-8; P. Ruetschi & R. T. Angstadt, "Anodic Oxidation of Lead at Constant Potential," J. Electrochem. Soc., 111 (1964) pp. 1323-1330; P. Ruetschi & B. D. Cahan, "Anodic Corrosion & Hydrogen & Oxygen Overvoltage on Lead & Lead Antimony Alloys," J. Electrochem. Soc., 104 (1957) pp. 406-412] recognized the phenomenon of lead corrosion in positive plate grids of lead-acid batteries over thirty years ago. From their studies of lead corrosion rates, they concluded that corrosion rate was minimized under float conditions where positive plate polarization was fifty (50) to two hundred (200) millivolts. Willihnganz [E. Willihnganz, "Accelerated Testing of Stationary Batteries," Electrochem. Technology, 6 (1968) pp. 338-341] supported this conclusion with elevated temperature accelerated testing of batteries. Willihnganz concluded that maintaining positive plate polarization at fifty (50) to one hundred (100) millivolts maximized plate life.
A more recent study by Reid et al (D. P. Reid & I. Glassa, "A New Concept: Intermittent Charging of Lead Acid Batteries in Telecommunications Systems," Proceedings of INTELLEC 1984, pp. 067-072) indicates that for certain starved electrolyte lead-acid batteries, ("starved electrolyte batteries" are batteries containing less electrolyte than the battery capacity for electrolyte. In the context of "starved electrolyte batteries", the battery capacity for electrolyte is the amount of electrolyte which can be absorbed by the battery plates and separators.) intermittent or cyclical charging, rather than constant float charging, extends battery life. Reid et al apparently did not look for optimum float or charging voltages or the effect of positive plate polarization on grid growth.
Industry practice has been to float battery systems at positive plate polarizations of fifty (50) to one hundred (100) millivolts to provide optimal battery life.
The ability to control positive plate polarization depends on the ability to control polarization at the negative plate. At the time of the early studies on lead corrosion as a function of positive plate polarization, when batteries contained lead-antimony alloys in the negative plate grid, the antimony depolarized the negative plate. Thus, during float positive plate polarization could be fixed by setting overall cell voltage since negative plate polarization was predictably zero (0) or was very small. When lead-antimony grids were used, practice was to float cells at an overall voltage of 2.15 volts per cell.
Problems with lead-antimony systems, such as antimony migration, led to adoption of lead-calcium alloys for battery grids. Since calcium, unlike antimony, does not depolarize the negative plate, polarization of the negative plate when lead-calcium alloys were used was unpredictable with respect to the overall cell voltage or cell current during float. Behavior of the negative plate in early lead-calcium alloy containing cells varied greatly from plate to plate, depending on factors such as method of fabrication and types of organic additives in the plate.
Uncertainty about the polarization behavior of the negative plate led to use of higher overall cell potentials to ensure adequate positive plate polarization to allow for some unknown negative plate polarization). For batteries in telephone switching systems, the industry increased the overall float potential to 2.17 volts per cell.
Present batteries having lead-calcium alloy grids do not have the problem of unpredictable negative plate polarization. These batteries are depolarized at the negative plate; this is achieved by including an appropriate amount of platinum in the negative plate. For float conditions of interest for lead-acid batteries, negative plate polarization now is zero (0) or acceptably small due to platinum's depolarizing effect on the negative plate.
Telephone industry practice is to float an overall cell potential of 2.17 volts. Thus, the cells are maintained at positive plate polarization of about seventy (70) to eighty (80) millivolts or more depending on the amount of platinum at the negative plate. This positive plate polarization is in accordance with the accepted belief heretofore, as set forth by Willihnganz, et al. noted above.
The standard potential of a lead-acid cell is 2.059 volts. The difference between 2.170 and 2.059, namely 0.111, is the total polarization present in the cell. When the positive plate is at 0.080 volts (80 millivolts) polarization, the remaining polarization, namely 0.030 volts, is at the negative plate. The amount of platinum present is controlled during manufacture, by design, to permit this amount of negative plate polarization.
Telephone switching systems operate in a narrow range of voltage from about forty-five (45) to about 53.5 volts. Batteries supplying stand-by power for telephone switching systems must provide power in this voltage range for the switching systems.
Voltage supplied by a battery is the sum of the voltage for each cell, for the cells in series in the battery. Thus, the voltage range of the telephone switching system can be used to calculate a voltage range per cell in the battery.
While a telephone switching system may impose limitations on acceptable voltage output range per battery cell, the cells may be able to deliver current over a wider voltage range. For example, if a battery contains twenty-four (24) cells in series and the low voltage limit for the telephone switching system is 45.12 volts, the low voltage limit for the telephone switching system is 1.88 volts per cell, whereas the cell may be capable of providing adequate power at much lower cell voltages.
Lead-acid wet cells often have a design end discharge voltage of about 1.75 volts per cell. The "design end discharge voltage" is the lowest cell voltage at which the cell can continue to supply a preselected constant discharge current. The difference between the lowest voltage acceptable by the telephone switching system and the design end discharge voltage for the cell represents available but unused battery cell capacity. For example, a battery providing 110 amp-hours over a three hour discharge period and having an end discharge voltage of 1.85 volts per cell at the end of three hour discharge period may be able to provide a 120 amp-hour over the same three hour discharge period if an end voltage of 1.80 volts per cell is acceptable. Thus, it is desirable to reduce the end discharge voltage per cell corresponding to the lowest voltage the telephone switching system can tolerate, in order to use more of the battery's designed capacity.
The upper voltage limit of the telephone switching system limits maximum charging voltage for the battery. As with the lower voltage limit, this maximum voltage limit for the telephone switching system can be translated to a maximum voltage limit per battery cell. If the maximum voltage tolerable by the telephone switching system is 52.08 volts, then a battery with twenty-four (24) cells in series has a maximum voltage per cell of 2.17 volts.